It is well known in the automobile and other industries to manufacture a load bearing structural frame comprised of tubes, and to subsequently weld sheet metal panels to the tubes. The welding of sheet metal to a closed tubular structure is commonly achieved via a single-sided spot welding process for the reason that the closed tubular structure does not permit the use of opposed acting electrodes to pinch together the metal surfaces as is the case of traditional two sided resistance spot welds. In the single-sided welding process, the sheet is placed against the tube, with the tube resting upon a back-up electrode. The welding electrode is then placed against the sheet metal. Weld current is conducted through to the electrode and the current passes through the sheet, and through the tube to the backup electrode that underlies the tube. A weld nugget is formed beneath the weld electrode, to weld the metal sheet to the adjacent surface of the tube.
It is characteristic of the single-sided welding process that the electrode must be applied to the sheet metal with considerable force so that the sheet metal and the tube are forcibly pressed together during the application of weld current in order to form a high quality spot weld between the sheet metal and the tube. It is accordingly necessary and desirable that the tube have metal thicknesses sufficiently stiff to resist bending thereof by the force applied by the electrode. This resistance to bending is particularly useful to obtaining high quality welds in the case of the single-sided welding of a sheet and tube where one of both of the sheet and tube is a galvanized metal. In the heat cycle of the welding of galvanized metals, the zinc element of the galvanized coating enters into the grain boundaries of the iron and the zinc remains in a liquid state at a temperature below the re-solidification temperature of the iron so that the zinc and iron may be at different conditions of re-solidification upon occurrence of unbending of the tube wall when the electrode pressure is removed at the end of the weld cycle.
It would be desirable to provide improvements in the afore described method of single-sided resistance spot welding a sheet to a tube in order to facilitate formation of high quality welds even without necessitating the use of heavy gauge tube wall thicknesses.